In a transistor using a nitride semiconductor, high withstand characteristics are obtained, due to electrical material characteristics of the nitride semiconductor that is a material with a wide band gap, and such a transistor becomes a semiconductor device that replaces a power device using silicon.
In addition, since the nitride semiconductor has a high electron mobility, the transistor can electrically perform a high speed response, and when the transistor is used as a high speed switching device, a high energy conversion efficiency is obtained.
The transistor using the nitride semiconductor like this has a high breakdown voltage and high speed responsiveness, and thereby it is expected that the transistor like this is used for a power conversion device such as a power source. Particularly, since a MOS type field effect transistor (MOSFET) having a gate structure made of a metal, an oxide (or an insulator), and a semiconductor can apply a sufficient voltage to a gate electrode, the MOSFET is a transistor structure advantageous in increasing speed.
However, the MOS type field effect transistor (MOSFET) using a nitride semiconductor has a low threshold voltage, and it is difficult to control the threshold voltage thereof to an arbitrary value. Accordingly, improvement of controllability relating to the threshold voltage of the MOSFET using a nitride semiconductor is desired.